Methods are known for operating an alarm system having a control centre and at least one control module which uses a two-wire line, used as a field bus, to provide a plurality of subscribers with a power supply voltage and to send communication messages in the form of pulse trains impressed on the power supply voltage, wherein each subscriber takes the power supply voltage and produces an internal operating voltage which is lower than the power supply voltage.
Alarm systems are known which have a control centre and at least one control module to which subscribers are connected by means of a two-wire line, said subscribers receiving from the control module, via the two-wire line operated as a field bus, both a power supply voltage and communication messages in the form of pulse trains impressed on the power supply voltage as voltage modulation, and each subscriber comprises a constant current circuit which is connected to the two-wire line and which is used to charge a storage capacitor which has a voltage regulator connected to it which produces an internal operating voltage.
Alarm systems of the type cited above are prior art. The two-wire line to which the subscribers are connected in parallel may have a length of between 1000 and 2000 m, for example, and is frequently routed in a ring shape, i.e. it starts and ends at the control module. The two-wire line is also referred to simply as a field bus or signalling line and the control module is also referred to as a bus master.
The subscribers may be sensors, e.g. fire or burglar alarms, and/or actuators, such as light-signal or sound-signal generators. The power supply voltage for the subscribers may be in the range from 20 to 40 volts, for example, at the start of the two-wire line.
The communication between the control module and the subscribers is handled on the basis of a digital communication protocol. The communication protocol defines time slots or time windows which are used to transmit pulses and pulse trains as data messages which represent addresses, commands and reports, in particular. Depending on the meaning assigned to them, the pulses may comprise starting pulses with a length of 1 ms, for example, synchronization or separating pulses with a length of 0.5 ms, for example, and pulse trains representing bit-coded messages, with a single pulse length of between 100 and 200 μs, for example.
The control module transmits the pulses and pulse trains to the subscribers by lowering the level of the power supply voltage for the length of the respective pulse, that is to say in the form of voltage modulation of the power supply voltage. In this case, the voltage swing or the modulation depth of the pulse trains embodying bit-coded messages may be a few volts, while the “long” starting and/or separating pulses may have other levels, e.g. 0 volt and/or +3 volts. The subscribers usually respond with appropriate current modulation.
The subscribers use an internal operating voltage which each subscriber derives from the power supply voltage or line voltage. In order to ensure that the power supply voltage or line voltage does not dip to a great extent when the signalling line is initialized or owing to a high current requirement for an individual subscriber in the course of operation, an internal storage capacitor in each subscriber is charged via a constant current circuit which limits the current to a few hundred μA, for example. The storage capacitor has a voltage regulator connected to it which provides the internal operating voltage of usually +3.3 V which the microcontroller and most other circuits in the subscribers use, except for actuators or actuator circuits, which draw their supply of voltage or current directly from the storage capacitor.
Each subscriber has a communication interface connected to the two-wire line, e.g. a UART interface for its microcontroller, which microcontroller detects and processes the pulses and pulse trains.
In connection with the manner of obtaining the internal operating voltage of the subscribers which is described above, this communication method has the drawback that not only the power supply voltage but also, in particular, the voltage swing which embodies the voltage modulation, and which may be 10 V, for example, at the start of the two-wire line, becomes increasingly smaller towards the end of the two-wire line on account of the resistance of the two-wire line. This can be attributed to the fact that the constant current circuits of the subscribers load the two-wire line or the power supply voltage only during the pulse pauses, because during the pulse length the power supply voltage is below the value of the voltage to which the storage capacitor of each subscriber has been charged as a voltage buffer, that is to say buffers the line voltage that is lower during the pulse length. In other words, the power supply voltage source and the two-wire line are loaded only during a pulse pause, because the subscribers are fed from their internal storage capacitor during the pulse length.
Towards the end of the two-wire line, the voltage swing can therefore become so small—on account of the loading of the two-wire line, which increases with the number of subscribers, in conjunction with the resistance of said two-wire line—that the subscribers in question are no longer able to detect the edges of the pulses correctly, since the gradient of the pulse edges also decreases as the length of the two-wire line increases. A further cause of communication errors which increase with the length of the two-wire line is that an additional current requirement for a subscriber, e.g. when an actuator circuit is connected, likewise results in a sudden decrease in the power supply voltage. This fall in the power supply voltage can be erroneously detected as the falling edge of a pulse by one or more subscribers.